It is thought he was killed with his two mothers, Jennifer and Sarah Hart, and five siblings when their SUV plunged off a scenic California highway and plunged 100ft into the ocean.

The bodies of the Harts and three of their adopted children have been recovered.

Investigators are still searching for three more children, including Devonte, who they believe were also in the car at the time.

"We know that an entire family vanished and perished during this tragedy," Tom Allman, the local county Sheriff, told reporters.

Devonte Hart and two of his siblings are missing after their siblings and parents were killed when their SUV plunged off a California cliff.Credit:
Steve Dykes

He said that the scene was unusual because the vehicle had crossed a 75ft stretch separating the road from the cliff edge and no skid or brake marks were found at the scene.

The sheriff added that there was "no evidence and no reason to believe this was an intentional act" and appealed for anyone with knowledge of the family's whereabouts before Monday to come forward.

The "Hart Tribe", as the married couple and their six adopted children were known, regularly travelled across the US offering people hugs and promoting unity at festivals and events.

Last year the family fulfilled their dream of moving onto a rural property in Woodland, rural southwest Washington State.

The Hart family at a Bernie Sanders rally in Washington State in 2016.Credit:
Tristan Fortsch

The family, who are believed to have left their home for a short trip, were recently visited by child protective services, a spokeswoman for the state's Department of Social and Health Services said.

The family's neighbours, Bruce and Dana DeKalb, told Sky News that they called child services on Friday out of concern for Devonte, who had come to their home to ask for food over the last week.

The DeKalbs also said that three months after the family moved to the area, one of the girls rang their doorbell at 1.30 am. "[She] was at our door in a blanket saying we need to protect her. She said that they were abusing her," Mr DeKalb said.

A helicopter hovering over the scene where a vehicle plunged off a cliff.Credit:
California Highway Patrol

Washington State child protective services opened an investigation and had tried to make contact with the family three times since Friday, Sky News reported.

The Harts were known to embark on spontaneous road trips and friends believe they may have been on their latest trip when the incident occurred.

The crash killed the 39-year-old parents, and their children Markis Hart, 19, Jeremiah Hart, 14 and Abigail Hart, 14.

Their three other children, Devonte, Hannah Hart, 16, and Sierra Hart, 12, are still missing.